1. Field of the Invention
The present invention relates to load elevators and methods of using them that can maintain the top of a variable load at a convenient predetermined height, and more specifically to a self-adjusting load elevator that can safely be used with pallet trucks.
2. Description of Related Art
In the handling of a number of packages or other objects, a common task is to manually transfer them between an elevated table, or conveyor, etc. and a pallet resting on the floor. While the table or conveyor remains at a fixed height, the top of the load on the pallet, where the next box or object is to be loaded or removed, usually is found at a height that varies as the packages, etc. are piled on or removed from the pallet. This difference in height, and the changes in this difference during loading or unloading of the packages, can be fatiguing and even harmful for a person doing the moving. In my U.S. Pat. No. 5,299,906, I disclosed a captive-air self-adjusting pneumatic load elevator that can be loaded or unloaded incrementally by humans or machinery while maintaining the top of the load at a preferred height above the floor.
Generally loads are put on pallets so that a worker with moving machinery can quickly remove a pallet with a full load from the load elevator, or place a pallet with a full load on it. However, when removing a pallet with a full load from the load elevator, the elevator will automatically rise from its minimum height (about 8 inches) to its maximum height (about 30 inches), a distance typically about 22 inches, with a force comparable to the weight of the load, typically somewhere in the range of 500 to 5,000 lbs. Similarly, the elevator will automatically descend about 22 inches if a full pallet of comparable weight is loaded onto an empty, but pressurized elevator. Therefore, the machinery for transferring a loaded pallet cannot simply slide it horizontally off or on the load elevator: it must first be able to lift or lower the loaded pallet vertically about 22 inches before moving the pallet horizontally. For this reason, expensive lift trucks are typically used to remove or load full pallets on self-adjusting load elevators.
Inexpensive pallet trucks, which can only be adjusted a few inches in height above the floor, cannot safely be used to remove a loaded pallet from such a self-adjusting load elevator. Moreover, the top surface of the pneumatic load elevator described in the above-referenced patent has a minimum height of about 8 inches, which is too high off the floor to accommodate a pallet truck. Thus, to accommodate pallet trucks, such a load elevator might be placed in a pit of depth equal to the elevator's minimum height, so that, when the elevator descends to its minimum height, the top of the load platform of the elevator is flush with the floor. However, before this can be done, a number of problems have to be solved.
For example, the load elevator of U.S. Pat. No. 5,299,906 is fitted with a circular, rotatable load platform so that workers can easily rotate a pallet loaded on it, but the bottom framework that supports the load platform is generally rectangular and therefore will probably be recessed in a rectangular pit. In such case, when the fully loaded elevator descends to be flush with the floor, there will be a dangerous gap in the floor between the rectangular pit opening and the framework for elevator's circular rotatable load platform.
Moreover, if the rotatable load platform on the elevator is instead made generally rectangular to match the rectangular pit opening, any rotation of the rectangular platform by the workers while using the load elevator will misalign the load platform with the pit opening, thus preventing the elevator from descending into the pit.
In addition, a pit-mounted load elevator presents several potential dangers to the workers who use it. For example, if the elevator load platform is rotatable, when it is flush with the floor a worker may inadvertently step on the platform as if it were part of the floor, only to have the platform rotate his foot out from under him. Therefore, there must be found some reliable way to prevent rotation of the load elevator platform when it is flush with the floor.
Also, assuming the elevator load platform is shaped to fit precisely in the opening of the pit, when the platform descends it may strike and seriously injure any portion of the foot of a worker that protrudes over the opening of the pit. Therefore, some reliable means for warning the worker of such danger should be found.
Finally, there is also a danger that, once a loaded pallet is removed off a descended load elevator, the elevator will automatically rebound out of the floor, knocking over the worker. Therefore, some reliable means of disabling an empty, descended elevator from rising automatically needs to be found.